Reliable, and thus top (5)
I believe the answer is:
solid
'reliable' is the definition.
(I've seen this before)
'thus top' is the wordplay.
'thus' becomes 'so' (synonyms).
'top' becomes 'lid' (lid is a kind of top).
'so'+'lid'='SOLID'
'and' acts as a link.
(Other definitions for solid that I've seen before include "Concrete or reilable" , "Three-dimensional shape, not hollow" , "thick" , "Firm - substantial" , "from one getting pushed around" .)